The present invention relates to an improved sampling system and method for obtaining representative samples of particles from concentrated aerosols, and, more particularly, to an improved sampling system and method for measuring the size and distribution of particles comprising aerosols in which the particle concentration of the aerosol is greater than 10.sup.6 particles/cm.sup.3.
There are two basic approaches for measuring the size of particles entrained in a gas stream, i.e., within the aerosol. One approach is to measure the size and size distribution of the entrained particles directly in situ without actually removing a sample from the gas stream. The second approach, and the approach to which this invention is directed, is to sample the particles from the gas and then obtain the size measurement. It is critically important, however, that the particles sampled for measurement are no more or less agglomerated than those remaining in the gas, otherwise the size and size distribution values will not be representative. A highly hostile, heavily particle-laden gas stream refers herein to a gas stream having a particle concentration greater than about 10.sup.6 particles/cm.sup.3 and a temperature which can range up to 1000.degree. C. or higher and/or a corrosive chemical environment. A highly hostile, heavily particle-laden gas stream offers significant problems in connection with obtaining a representative sample for measurement and is undisclosed in the prior art.
For example, U.S. Pat. No. 3,478,600 describes an apparatus designed for isokinetic sampling in a laminar flow stream, i.e., the gas/particle mixture enters the receiver tube with the same velocity as the process line flow. Turbulent process flow and highly concentrated aerosols would clog the system. A 2:1 dilution ratio is maintained; and although further dilution is possible in the expanded chamber, changes in the particle size distribution may have already occurred.
U.S. Pat. No. 4,361,028 describes a system for measuring the quantity of particulates being discharged in the exhaust of a vehicular internal combustion engine. The exhaust gases are introduced into a dilution tunnel where they are mixed and diluted with clean air. A part of the diluted gas is then sampled by means of a sampling probe, and the sample is carried to a sampled gas line in which a filter is provided to collect particulates contained in the sampled gases. The quantity of particulates being discharged in the engine exhaust gases is computed on the basis of the time differential of an electric signal from a differential pressure transducer which detects the pressure drop across the filter. Nevertheless, particle size and size distribution of the sampled gas stream is not revealed and does not appear to be a factor affecting operation of the described measuring system.